This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Comparative genomic analysis is used to study the evolution of immune system genes in response to differences in life history strategies, specifically differences in suites of parasites experienced by landlocked versus sea run fish. Sea run Atlantic salmon encounter parasites in two habitats, while landlocked fish only encounter parasites in one habitat. It has been hypothesized that greater genetic diversity of immune system genes should exist in populations experiencing more parasites. This project will assess genetic variation in the Major Histocompatibility Complex (MHC) Class II B gene in populations of sea run and landlocked Atlantic salmon. It will assess the type of selection acting on the MHC Class II B gene and identify corresponding amino acid changes in the Antigen Binding Site (ABS) of the MHC Class II B gene. This will provide information on the amount of functional diversity in each population, enabling a correlation between functional diversity and parasite exposure. This study will be expanded to other immune system genes: MHC Class-II A, MHC Class-I, and Transporter Associated with Antigen Processing (TAP). This will allow multilocus genotyping of fish, and determination of the existence of unique combinations of immune system genes. The goal is to determine if exposure to different numbers of parasites correlates with levels of MHC polymorphism. This investigation of MHC diversity will help understand how an organism's immune system can be exploited for better protection against parasites.